Porous ceramic insulating material



Patented Dec. 1, 1942 v I I UNITED STATES PATENT OFFICE I 2,303,964 POROUS CERAMIC INSULATING MATERIAL Alfred Ungewiss, Berlin-Grunewald, Germany;

vested in the Alien Property Custodian No Drawing. Application April 5, 1938, Serial No. 200,269. In Germany April 20, 1937 8 Claims. (Cl. 106-46) This invention relates to a porous ceramic inponents of the novel mass in a highly advansulating material showing a low dielectric loss tageous manner it they are in preliminarily baked and to a process of producing same. or electrically molten condition.

v After having been fired the material accord- To increase strength, it the batch contains an ing to the invention can be subjected to methexcess of free silicic acid, itis advisable to add ods of treatment usually applied in wood and a basic component an if it contains free basic metal working, i. e., turning, drilling, milling or oxide, an acid component. sawing, with corresponding dimensional accu- Such basicoxides, for instance MgO o .CaO, racy. This subsequent workability of the me.- are preferably used in the form of their carterial permits a variety of new uses for which 10 bonates, as magnesi r Cale-spar r d010- ceramic materials were not suited hitherto. mite, since these substances at Seger cone 10 Until now, ceramic insulating materials poor to give on their carbonic acid, whereby loosenin electric loss comprised merely vitrified subing of. the structure and a further improvement stances of various compositions, as magnesium of workability are attained. I silicates of the steatite group, which contain ti- 15 The relatively slight plasticity of such mix tanium dioxide as chief constituent, and others. tures resulting from this composition frequently All these substances, however. lack workability requires, however, additional plasticization which and for this reason cannot be made after firing should absolutelyavoid the addition of constitinto pieces 'of accurate measurements. This fact uents rich in alkali to retain the property of prevented their use for numerous purposes. as low dielectric loss. molded pieces meeting substantial requirements Suitable plasticizers are substances free from as to accuracy of measure cannot be produced by alkali and containing clay substance, as kaolin ceramic firing as such due to the action or dryand clay. particularly calcium bentonite.

I ing and firing shrinkage and distortion during If naturally occurring magnesium silicates like baking. 2'5 soapstone are used as mass constituents, their I The insulating substances according to the inintroduction will simultaneously effect additional vention consist of at least 50% technically pure plasticizati on of the mass in a highly advantaacid or basic oxides or mixtures of such oxides geousmanner. Plasticizers that can be used in in original or molten form. or, alternatively, of each instance are the pure hydroxides oi the mixtures of these products that are either pre- 3 oxide components employed or mixtures thereof. liminarily treated to form oxides or molten, par- The known organic plasticizers, on occasiommay ticularly silicates and spinels rich in base, as also be used to improve the plasticity of the forsterlte or magnesia spinel, and. besides, of the masses, particularly if degrees of plasticity are usual constituents of ceramic masses, as plastito be attained which permit deformation of the cizers and inorganic binding substances. masses on a press for wire cutting and by turning When making up the batch of these constitor by moist or dry pressing. uents, it should be noted that eutectic mixtures An essential condition of the success of the of the constituents, which would melt at the usual "new process is to avoid the presence of eutectics industrial baking temperatures or pyrometric that will melt at the baking temperatures emcone 10 to 15, should be avoided. Firing is there- 40 played. Particularly in masses consisting of fore efiected at these customary temperatures MgOA12OaSiO2 should the known low-melting without any appreciable sintering together of the mixtures be absolutely avoided or. if at all, should constituents, so that a porous body of fine be present only in slight quantities. grained structure is obtained which possesses the When plasticizers containing clay substance are properties mentioned above. used, the formation of greater amounts than 10% As basic oxides those of the alkaline-earth and of the eutectic clay substance-'magnesium siliearth metals are suited; as acid oxide, silicic 'cate must also be avoided. acid in various forms may be used. By suitable composition of the mass or the Masses which contain as a component mixtures addition of organic substances free from alkali oi the two-component system MgO-SiOz, which 60 and burning without residue the dielectric conhave been preliminarily baked at high temperastant oi the substances may be varied at will ture or molten in the electric arc, possess special between 2.5 and 7. 10

- st ength. Mixtures of the mixed crystal series Working of the substances prepared in the magnesia-magnesia spinel, which are perfectly manner described and distinguished by very slight miscible in any proportion, can also serve as comdielectric loss may be eflected at speeds 20 to the subthe invention are of course those 01 densely sintered substances, but by the strength increasing measures described and the systematic production of a comparatively slight degree of porosity adapted to the working strength desired the flexibility of the substances can nevertheless be brought up to that of a medium grade porcelain.

The following examples state a few compositions of masses according to the invention and their dielectric data:

Example 1 Parts by weight Alumina 60 Molten silicic acid l Magnesium hydroxide (pasty) 10 Aluminium hydroxide (pasty) 10 Clay substance Calcium-bentonite 5 This mass after dry pressing at a bakingtemperature of Seger cone 13 disclosed a water absorbing capacity of about 20%; the dielectric loss amounted to tg =15.10 ()\=300), dielectric constant =3.3.

Example 2 Parts by weight Molten silicic acid 40 Silicic hydrate in gel form 30 Magnesite 20 Calcium-bentonite This mass after dry pressing at a firing temperature of Seger cone 13 disclosed a water absorbing capacity of 35%; the dielectric constant is E=2.6; the dielectric loss is tg =16.10- (=i300) dielectric constant =3.3.

the junctions, so that the these in themselves porous sub;

Example 3 r Parts by weight Molten quartz 40 Forsterite, electrically molten 20 Magnesite 5 Silicic hydrate in gel form 25 Calcium-bentonite 10 This mass after dry pressing and baking at Seger cone 13 disclosed a water absorbing capacity of about 30%; its dielectric constant is e=2.8; dielectric loss tgw=15.l0 (i=300); the bending strength amounts to an even one-third of the usual porcelain value.

Example 4 Parts by weight Magnesia spinel, electrically molten, of the composition 2MgO-1Alz0a 10 Forsterite, electrically molten 10 Magnesite 2'1 Plastic clay 'l Calcium-bentonite 6 Organic plasticizer 3 This mass after dry pressing at Seger cone 13 disclosed a water absorbing capacity of about 25%. Its dielectric loss is tgw=1.10' (i==300); dielectric constant e=3.5.

Example 5 Parts by weight Forsterite, electrically molten 50 --.---Silici'c-ac id, electrically molten 15 Magnesite'--i-. 22 Plastic clay 11 Calcium-bentonite 2 This mass after baking at Seger cone 13 disclosed a water absorbing capacity mately 10%; its dielectric loss is tU'y==3-4.10- ()\=300) dielectric constant e=5. The bending strength of this substance is equal to that of a pressed porcelain. It will be noted that in the above formulae in each case there is at least one of the ingredients of the three groups mentioned in proportions not greater than of the order of 0%. In Example the chief ingredient is alumina while the silica and magnesia are both low. In Examples 2 and 3 the silica is high, while the alumina is quite low. In Example 4, the magnesia and alumina are more nearly equal, but the silica is present in relatively small amounts incidental to other compounds, while in Example 4 the silica and magnesia are in relatively even proportions while the alumina is introduced only as an incidental ingredient of the plasticizer. Each of these examples results in avoiding any material amounts of low melting compounds.

The substances according to the invention can be used in many-ways and employed to particular advantage in the manufacture of pattern parts.

In case of difiicultly moldable articles they may be made of several parts which can be conveniently molded separately and are subsequently glazed together. This method is an essential feature of the invention as is also the step in the manufacture of insulating bodies of dense ceramic material which require particularly accurate working after firing at certain points to attach to these points portions of the workable substance according to the invention by glazing. Glazing together is effected at much lower temperatures than the firing temperature and does not in any way interfere with the accuracy of shape of the articles. In connection with glazing further working is possible also.

The invention is not restricted to the forms of application and the compositions of masses described, but may be varied in many ways without departing from its fundamental idea.

I claim:

1. A ceramic insulating body for use in high frequency engineering, said body consisting of grains sintered together only at their contacting points and constituting a finely porous structure in which the pores constitute at least 10% of the bulk, said body being workable by the usual tools at the usual speeds in turning, fraising and boring metal, having a dielectric loss of to a of less than 2010-:300 m, and a dielectric constant of between 2.5 and 6, and having the composition resulting from firing together ceramic materials, at least 50% of said materials being of the group consisting of magnesia-alumina spinel and forsterlte, approximately 13% of said material being alkali-free plastic clay, and the remainder of said materials being prefused oxide of alkaline earth metal.

of approxithe group consisting of magnesia-alumina spinel and forsterite, approximately 13% of said materials being alkali-free plastic clay and the remainder 01' said materials being prefused material oi the group consistingoi the oxides alkaline earth metals and silicon.

to melt anyportion oi the body, the grains being compacted and tired so that at least'%' of the bulk of the body consists oi pores.

6. A method of making a ceramic insulating body which consists in forming a batch, at least 50% oi the batch being of the group consisting of magnesia-alumina spinel and 'forsterite, approximately 13% of the batch beingalkali-free plastic clay, the remainder of the batch being of ceramic material consisting of preiused material ot the group consisting of oxides of alkaline earth metals and silica, and as 'much organic plasticizer as necessary to make the mass workable, forming the material into shape, the batch the s. A ceramic body for use in high' frequency engineering, said body consisting of the fired combination of materials substantially as follows: 50 parts by weight magnesia .spinel, electri-' cally molten, of the composition ZMgO-lAlaOa, 10 parts by weight forsterite, electrically molten,

27 parts by weight magnesite, 7 parts by weight plastic clay, 6 parts by weight calcium-bentonite, 3 parts by weight organic plasticizer, and the batch being fired at temperatures between cone l0 and 15 for a time that results in a finely porous structure in which the total volume of the pores constitutes at least 10% ofthe total bulk.

engineering, said body consisting of. the fired combination oi materials substantially as follows: 50 parts by weight forsterite, electrically molten, 15 parts by weight silicic acid, electrically molten, 22 parts by weight magnesite, 11 parts by weight plastic clay, 2 parts by weight calciumbentonite, and the batch being fired at temperatures between cone 10 and 15 for a time that 4. A ceramic body ior use in high frequency containing no materials which melt at or below cone 10, firing the body. at a temperature between cone 10 and 15 at a point lust sufllcient to cause the grains of the material to unite at their points ,oi'suriace contact by crystalline growthand not to melt any portion of the body. the grains being compacted and fired so that at least 10% of the bulk of the body consists of pores.

\ 7. A. method of making a ceramic insulating body which consists in forming a batch having approximately the following compositions: 50

parts by weight magnesia spinel, electrically molten, of the composition ZMgO-lAliOa, 10

parts by weight forsterite, electrically molten, 27 parts by weight magnesite, 7 parts by weight plastic clay, 6 parts by weight calciumbentonite,

- 3 parts by weight organic plasticizer, forming the material into shape, the batch containing no results in a finely porous structure in which the total volume oi the pores constitutes at least 10% of the total bulk.

5. A method oi making a ceramic insulating body which consists in forming a batch, at least of the batch being of the group consisting of magnesia-alumina spinel and forsterite,. approximately 13% of the batch being alkali-free plastic clay, the remainder of the batch being of ceramic material consisting of prefu'sed oxide of materials which melt at or. below cone l0, firing the body at a temperature between cone 10 and i5 and to a point just sufiicient to cause the grains oi the material to unite at their points of surface contact by crystalline growth and not to melt any portion oi'-the body, the grains being compacted and fired sothat at least 10% of th bulk oi the body consists of pores. 1 8. A method. of making a ceramic insuiatin body which consists in forming a batch having I approximately the following compositions: 50

parts by weight forsterite, electrically molten, 15 parts by weight 'silicic acid, 'electrically' molten, 22 parts by weight magnesite, 11 parts by weight plastic clay, 2'parts by weight calciumalkaline earth-metal, and as much-organic plasticizer as necessary to make the mass workable,

, forming the material into shape, the batchcontaining no materials which melt at .or below cone 10, firing the body at a temperature between cone 10 and 15 to a point just suificient to cause the grains of the material to unite at their points of surface contact by crystalline growth and not bentonite, forming the material into shape, the batch containingno materials which melt at or below cone 10, firing the body at a temperature between cone 10 and 15 at a point Just suificient to cause the grains oi the material .to'iinite at their points of surface contact by crystalline growth and not to melt any portion of the body.

the grains being compacted that at least 10% of the bulk of thebody oi;-

oi-es.v v a Y sum UNGEWISS. 

